1. Field of the Invention
The present invention relates to a technology for correcting an optical distortion that occurs in an optical system image.
2. Description of the Related Art
Conventionally, there are optical measurement apparatuses that perform measurement based on an optical image, such as an interferometer and a length measuring machine. In such optical measurement apparatuses, two-dimensional optical distortion is caused due to distortion in an optical system in imaging such an optical image. The optical distortion causes deterioration in measurement accuracy.
With such reason, a method of measuring the optical distortion and an apparatus for correcting measured optical distortion have been proposed. For example, in one of the method, a regular reference pattern, such as a tetragonal lattice, is captured with a charge-coupled device (CCD) camera, and the optical distortion is measured based on deviation from the reference pattern to measure is widely known.
Japanese Patent No. 2717407 discloses an apparatus for correcting optical distortion. The apparatus detects two-dimensional deformation of a moiré fringe caused by superimposing a deformed lattice that is obtained by projecting a reference lattice on a flat measurement object, removes the optical distortion measured.
Measurement of the optical distortion by the conventional technology is based on detection of two-dimensional deformation of an image pattern or a moiré fringe. However, since detection accuracy of such deformation is low, measurement accuracy for the optical distortion cannot be sufficiently high for performing highly accurate interference measurement and dimensional measurement.
In performing two-dimensional positional detection for an image pattern or a moiré fringe from an image optical image, the positional detection is performed based on a change in luminance in the image pattern or the moiré fringe. However, since brightness of an optical system is not uniform in the optical image, an offset component and a modulation component of brightness occur in the optical image.
Accuracy of two-dimensional positional detection for the image pattern or the moiré fringe is limited to a sub-pixel order by influences of these components. Therefore, measurement accuracy for optical distortion based on detection of two-dimensional deformation is also limited to the equivalent degree.